A school bus should be easy during a morning commute. Just follow one til it stops for kids.

If you speed past a cop he will turn the lights on just for you to take a pic

Go set a fire and wait for the fire truck to arrive. (not recommended but it works)

Hell if you shoot the annoying neighbor you will get a fire truck, ambulance and a cop all in one picture.

1 - that's an easy one.
2 - already covered
3 - haven't got that one, but I work on what seems to be a major thoroughfare for getting to all the fires on the north side of town during the day
4 - one of my annoying neighbors just might be a utility worker...

Couple other aces up my sleeve that I'm not going to give away just yet...